The present invention relates to a liquid mixing ratio detecting apparatus for detecting a liquid mixing ratio of a mixed liquid such as an apparatus for measuring a concentration of alcohol mixed in gasoline fuel, and more particularly to a liquid mixing ratio detecting apparatus having a pair of electrodes immersed in the mixed liquid to be utilized as a capacitor.
A prior art liquid mixing ratio detecting apparatus will hereinafter be described by taking an apparatus for detecting a mixing ratio of a mixture of gasoline and alcohol as an example. In foreign countries, gasoline fuel having alcohol mixed in gasoline (hereinafter abbreviated as alcohol mixed gasoline) is used as mixed fuel. Such mixed fuel has an air-fuel ratio different from pure gasoline fuel. As a result, there exist differences in a fuel injection amount and ignition timing therebetween.
In general, the fuel injection amount is obtained by multiplying a basic fuel injection amount by a coefficient corresponding to the air-fuel ratio. Further, the air-fuel ratio is generally about 15:1 for pure gasoline, and it is about 6:1 for 100% alcohol. As a result, there results the alcohol concentration versus air-fuel ratio characteristics in which the proportion of air decreases as an alcohol concentration increases. According to the alcohol concentration versus air-fuel ratio characteristics, in the case of alcohol mixed gasoline, it is necessary to change the air-fuel ratio in accordance with an alcohol concentration in the fuel tank and consequently to change the coefficient by which the basic fuel injection amount is multiplied.
As a liquid mixing ratio detecting apparatus for detecting a concentration of alcohol in alcohol mixed gasoline, there is known a system in which a pair of electrodes are immersed in the alcohol mixed gasoline and are used to detect electrostatic capacitance formed between the pair of electrodes (hereinafter referred to simply as "capacitance"). That is, since there is a great difference in the magnitude of dielectric constant between alcohol and gasoline (the dielectric constant of alcohol is 33.6 and that of gasoline is 2), a change of capacitance between the two electrodes depending on a content ratio of alcohol and gasoline is utilized. More specifically, as shown in FIG. 10, a pair of electrodes 1 and 2 are disposed in a pipeline 4 of alcohol mixed gasoline, for example, so as to be immersed in the alcohol mixed gasoline, and the electrodes 1 and 2 are connected to an oscillation circuit 3. An oscillation output of the oscillation circuit 3 is converted into a voltage (shown by Vs) through a frequency/voltage converter 5.
An example of a conventional apparatus employing the above-described system is disclosed in JP-B-2-103264. The liquid mixing ratio detecting apparatus disclosed in the above laid-open utility model application is designed so that the pair of electrodes 1 and 2 are used as a constituent element of a resonance circuit in the oscillation circuit 3, and it is arranged to detect a change of an oscillation frequency due to a change of electrostatic capacitance based on a change of dielectric constant caused by a change of a concentration of alcohol.
However, in the liquid mixing ratio detecting apparatus disclosed in the above laid-open application, there arises a problem in that, because metal ions and impurities are dissolved in the mixed fuel from inner wall members of a fuel tank and/or a pipeline thereby to change the conductivity of the mixed fuel, accurate detection is difficult to be performed. This is because that, in the sensor disclosed in the above laid-open application, as shown in FIG. 10 by dotted lines, a resistor Rf1, whose resistances value is determined by the conductivity of the mixed fuel and the structure of the electrodes 1 and 2, is present between the electrodes 1 and 2, and similarly a resistor Rf2 and a capacitors Cs are present between the electrodes 1, 2 and the pipeline 4, and thus metal ions and impurities dissolved in the mixed fuel act to reduce the resistance value of the resistors Rf1 and Rf2 and the capacitance value of the capacitor Cs, especially the resistance value of the resistor Rf1, and, as a result, it becomes difficult to obtain desired information only from the magnitude of the capacitance between the electrodes 1 and 2.
Besides, since the electrodes 1 and 2 in FIG. 10 are used as constituent elements of the resonance circuit of the oscillation circuit 3, in order to perform accurate detection, it is essential that the capacitor of the resonance circuit has a small leakage current. However, when the conductivity of the mixed fuel is increased, then Rf1 and Rf2 are decreased and hence a leakage current is increased, thereby to increase a detection error. Further, it is considered that, since oscillation conditions are not satisfied, a reduction in an oscillation output and even stoppage of oscillation occur, and, as a result, the detection becomes difficult to be performed.
The present invention is intended to solve the above-described problems, and it is an object of the present invention to provide a liquid mixing ratio detecting apparatus in which an electrode capacitor is not used as a constituent element of an oscillation circuit, but is used as a capacitor for determining an inclination rate of a differential waveform or an integral waveform, and besides which is constructed to reduce an influence of a leakage current on an inclination rate signal, thereby reducing an adverse influence of a change of the conductivity of the mixed fuel.